Generally, in an air conditioning system for vehicles, a compressor for compressing refrigerant is arranged to be driven by a vehicle engine through an electromagnetic clutch. The electromagnetic clutch is provided with an electric excitation circuit in which a grounded lead wire is connected to the body of the compressor. In the interior of some conventional vehicle air conditioning compressors, sliding parts are lubricated by a lubricating oil which is supplied from an oil stocker provided within the body of the compressor or which is separated from an oil suspended refrigerant flowing through the entire air conditioning circuit, including the compressor, distribution pipes, and a heat exchanger. In some other conventional compressors for air conditioning of vehicles, the sliding parts thereof are lubricated by a part of the refrigerant per se, which contains therein an oil component. However, when the conventional compressor is continually operated, regardless of the fact that a leakage of a lubricating oil or a refrigerant from the air conditioning circuit including the compressor occurs, there is a lack of lubrication for the sliding parts of the compressor. That is to say, if a leakage of lubricating oil occurs, a shortage of the lubricating oil takes place, thereby causing a lack of lubrication for the sliding parts of the compressor. Further, if a leakage of refrigerant occurs, a shortage of the refrigerant, which carries a lubricating oil to the sliding parts of the compressor, takes place, thereby causing a lack of lubrication for the sliding parts of the compressor. As a result, seizure of the sliding parts of the compressor eventually occurs, which brings about a breakdown of the compressor.